Method for non-destructive determination of mechanical properties of metal castings



Q Feb. 24, 1970 o STEIN 3,496,766

METHOD FOR NON-DESTRUCTI VE DETERMINATION OF MECHANICAL PROPERTIES OF METAL CASTINGS Filed July 5, 1967 3 Sheets-Sheet 1 IN VENTOR. DAV/D .5 TE/N POM/4 ER, IVA/055E MAFTE/VS ATTORNEYS.

Feb. 24-, 1970 D. STEIN METHOD FOR NON-DESTRUCTIVE DETERMINATION OF MECHANICAL PROPERTIES OF METAL CASTINGS Filed July 5, 1967 3 Sheets-Sheet 2 -P/V7AA/V7 Rf/gfREA/Cf STANDARD ure A/uMam fll/YSOU/VD/VESS) e INVENTOR. DAV/0 STE/N Feb. 24., 1970 D. STEIN 3,496,766

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DAV/0 STE/N FOWLER, /(/v055 5 M42 news A T TOENEVS'.

llnited States Patent 3,496,766 METHOD FOR NON-DESTRUCTIVE DETERMINA- T ION 0F MECHANICAL PROPERTIES OF METAL CASTINGS David Stein, Fountain Valley, Calif. (146 Marywood, Claremont, Calif. 91711) Filed July 3, 1967, Ser. No. 650,918 Int. Cl. G01n 3/00 U.S. Cl. 73-88 12 Claims ABSTRACT OF THE DISCLOSURE A nondestructive method of quantitatively determining mechanical properties of metal castings by nondestructively measuring the soundness of the casting in any critical area by penetrant inspection, testing a simultaneously cast testing bar of determinable soundness for a desired mechanical property and measuring the mechanical property in the critical area of the casting by reference to a correlation of soundness to said mechanical property for the alloys of said casting.

Background of the invention Quality control is a basic problem in most industries since the manufacturer must deliver products of consistent dimensions, properties and capabilities. In the manufacture of certain products, no special problems exist in obtaining homogenous and uniform products on a batch or continuous production basis. With those products statistical sampling and testing provides information which is acceptably representative of each and every product produced.

Many other products, however, vary greatly from one to the next, and random or scheduled sampling from a number of these products produced under apparently parallel conditions gives no assurance that any individual product will meet the necessary specifications for its intended service. Individual testing of each product is generally the only manner of assuring that such products are qualified for service, but such testing is feasible only if the product is not damaged by the testing.

Metal castings present serious quality control problems since the castings produced within a production run vary substantially in physical properties from one to the next and even from place to place within a single casting. Cast aluminum presents particularly difiicult problems because aluminum is a very light metal which has fairly high heat conductivity and also has a tendency to shrink on solidification. This combination of properties causes undesirable voids or defects because of the tendency to prematurely solidify in certain regions without sufficient hydrostatic pressure within the mold to fill the void.

To compensate for these effects, aluminum and its alloys are cast with molds providing for continuous gravity feeding of molten metal from secondary feeding stations known as risers to compensate for shrinkage, and differential zone cooling, e.g. chilling, is employed to control the progress and direction of solidification. However, variations in risering, chilling, gating (feeding) and pouring practices all affect the properties of the individual casting, as does variation in alloy composition. Moreover, aluminum possesses a high affinity for hydrogen and oxygen which increase both with temperature and with the time the alloy is in the molten condition, thereby introducing another factor which can vary the properties of the resulting casting.

It is apparent that the present state of the aluminum casting art is not capable of economically mass producing castings of statistically controllable or predictable properties. Furthermore, the tests from which the art 3,496,765 Patented Feb. 24, 1970 "Bee can select for determining mechanical properties such as yield strength, tensile strength, and percent elongation weaken or destroy the casting. Thus, one can only determine what the strength of the casting was, but not what it is.

Description of the prior art There has been much interest and development in nondestructive testing, but the prior art inspection procedures such as dye penetrant or radiographic inspection are only capable of indicating internal discontinuities or external flaws in a casting. There is as yet no acceptable nondestructive method for quantitatively determining mechanical properties of casting. All this is of considerable concern since cast metal and especially cast aluminum is being adopted for service where ductility and strength are of great importance.

It is therefore an object of the present invention to provide a nondestructive method for quantitatively deter mining the mechanical properties of a metal casting.

A further object of the invention is to provide a procedure for rapidly, accurately and reliably determining the ductility and other mechanical properties of each of a series of castings of the same or similar alloy in each of several various critical stress zones.

Another object is to provide a method of nondestructively determining acceptability of castings to assure quality in the mass production of castings.

Summary of the invention The present invention is based on the relationship of soundness or integrity of cast structure to mechanical properties. It has been found that with all other conditions, i.e. composition and process conditions, held constant or substantially constant, mechanical properties such as percent elongation E) as a measure of ductility will approach their highest values at maximum cast soundness and as cast soundness decreases, the mechanical properties degrade in accordance with a determinable relationship. For any second or other casting of the same alloy, the mechanical properties or percent of maximum mechanical properties obtainable at any known soundness follows this same relationship.

In a preferred embodiment of this invention, a coupon is cast with the main casting so that there is no substantial variation between the main casting and the coupon with respect to alloy composition, casting procedures, or initial molten alloy temperature, leaving integrity of cast structure as the only major or substantially varying factor. The mold is designed and the casting is conducted so that at least a portion of the coupon has a known in tegrity or soundness, preferably the maximum integrity practicably attainable. The main casting is nondestructively tested to determine its soundness in at least one critical zone, and the coupon is destructively tested to measure the strength or other critical mechanical property of its soundest portion. Using previously determined reference information which correlates changes in soundness with changes in mechanical properties, together with the determined soundness of the main casting, the known soundness of the coupon, and the coupon destructive test data, the strength of the main casting in the one or more critical zones is determined. The main casting is then graded or routed to one of a plurality of destinations in accordance with its determined strength.

Description of the preferred embodiment In an exemplary embodiment of the method, the coupon remains in attachment with the main casting during all treating processes to eliminate variances between th properties of the coupon and of the main casting caused by such process. Further, a portion of the outer skin of the main casting is removed by chemical milling, chemical etching, machining, electrolytic polishing, vibro-honing or any other method that removes the metal surface layer without plastically deforming the sub-surface layer so that its integrity as related to internal porosity can be nondestructively measured by dye penetrant inspection. Th skin of the coupon is preferably removed to eliminate any possible variations between the properties of the main casting and coupon which might otherwise result from the milling.

By means of the present invention, the properties of each of a series of castings are accurately determined leaving the casting unharmed. The present invention is able to provide these results since all that is required to be determined of the main casting or desired areas of the castings is soundness. With this information and information provided by secondary means, the mechanical strength is at Once established.

These and other objects and advantages of the invention will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view illustrating a casting of a missile fin exemplarily marked to indicate zones of acceptable common mechanical properties and having a coupon attached in accordance with the method comprising this invention;

FIG. 2 is a partial enlarged perspective view of the casting illustrated in FIG. 1;

FIG. 3 is a graph representing unsoundness in terms of increasing fluorescent penetrant reading as abscissa versus depreciation of mechanical properties as ordinate with two curves representing tensile strength percent, and provisional percent elongation, respectively; and

FIG. 4 is a graph of a family of curves representing percent elongation as the ordinate versus yield strength as abscissa.

The mechanical properties of any selected volume of a given casting are dependent on the chemical composition of the alloy, heat treatment, solidification rate and integrity of cast structure. It has been found that integrity of cast structure or solidification rate can both be measured by number and size respectively of the pattern revealed when the skin of the casting is removed and penetrant inspected under controlled conditions. By preparing a series of penetrant standards for a given alloy in different thicknesses, it has been discovered that the various patterns in terms of size and number of microvoids can be matched and the variables of solidification rate and integrity as a measure of soundness can be effectively correlated to the mechanical properties of the standards.

For a given casting process it has been discovered that the yield strength of sound areas of a casting is related to the section thickness of the matching penetrant standard and can be simply determined by multiplying a previously determined thickness factor for the standard by the yield strength of a maximum soundness test specimen removed from the simultaneously cast coupon of determinable soundness from numerous tests, tensile strength has been found to be correlated to soundness according to the invention in terms of a determined fraction of maximum difference between the yield and tensile strength of a maximum cast soundness standard related to the soundness of penetrant inspection standards. To determine the tensile strength of an area of the casting a penetrant standard is selected to match the pattern appearing on the area of the casting to be tested. The tensile strength fraction for that standard multiplied by the difference between the yield and tensile strength of the maximum cast soundness coupon added to the yield strength of the area determines the tensile strength of the area. A provisional percentage of elongation can be determined from a simple fraction of maximum relationship derived by destructively testing sample castings to establish the elongation fraction for each soundness as represented by a articular dye penetrant pattern. However, the actual ductility of the volume of the casting in question as measured by percentage elongation also depends on the precipitation hardening heat treatment and will only agree with the provisional percentage elongation if coincidentally the volume of the casting has been subjected to a heat treatment identical with that of the standard reference plate.

Since this is rarely the case, the provisional percentage elongation is adjusted to compensate for variations in heat treatment according to the following relationship:

where Y is yield strength, a is a constant and has a value of 0.83 for A357T6 aluminum alloy, in is a parameter and E is percentage of elongation.

This relationship was established from plots of the destructive test data of numerous cast samples subjected to different heat treatments utilized to establish a family of linear curves such as FIG. 4 utilizing the value of yield strength of the reference plates, the final adjusted percentage elongation is determined from these curves.

The reference data of Table I and FIGS. 3 and 4 is obtained by preparing a series of standard reference plates of a selected alloy, e.g. A357T6 aluminum alloy in thicknesses of 0.20, 0.50 and 1.00 inch. The plates are prepared from rectangular castings in each thickness which are poured and chilled along one edge so that there is variation in soundness across the length of each casting (as a result of variances in micro-shrinkage) from maximum soundness to the soundness corresponding to a brittle casting. Sample bars of different degrees of soundness are taken from along the length of each casting and are chemically milled to remove a 0.002 inch depth of metal. Milling is effected carefully to prevent preferential attack of the microconstituents utilizing a 5 percent hot caustic soda treatment followed by a water rinse, a 5 to 10 percent nitric acid treatment and a final water rinse. Inhibitors may be added to prevent differential attack of the microstructure.

The milled sample bars are immersed in a liquid bath of a dye dissolved in a water washable organic solvent or suspended as an emulsion in a liquid vehicle. Some of the dye in liquid enters the microvoid channels of the bar. The dye remaining on the surface is washed away with water and the sample bar is dried. The surface of the sample bar is then dusted with an adsorbent such as chalk which attracts the dye that has penetrated into the voids to the surface. The resulting patterns of dye are unique for each sample bar in each thickness and represent the number and size of the micropores which is related to the soundness of the particular sample bar. The dye can be of the type that is visible under ordinary illumination or of the fluorescent type that is visible under ultraviolet light. The sample bar can be photographed or a plastic replica taken of the pattern before being destructively tested, or the bar can be split and the remaining half destructively tested to determine the correlation between the degree of soundness of each bar as represented by dye penetrant pattern and the fraction or degree of the various mechanical properties. The sample bars in each thickness series are assigned consecutive numbers or other designation, and photographic or other reproductions of these reference standards are mass produced, if desired, for visual comparison to castings of the corresponding alloys.

The mechanical properties of each sample bar are determined by destructive testing. The values of the mechanical properties at maximum and at lower degrees of cast soundness are utilized to determine the fraction or degree of degradation from maximum of various mechanical properties, e.g. elongation, yield strength, and tensile strength for each sample bar to form a series of dye penetrant standard reference plates, The mechanical properties of any other casting of the same alloy or any part thereof having the same apparent soundness as evidenced by a characteristic dye penetrant pattern will have the same degree of maximum cast soundness properties as the.

comparable standard reference plate. In the case of an A357T6 alloy the value of yield strength at each thickness is utilized to determine the relationship of thickness of the reference plate to yield strength which can be plotted or presented in tabular form as in the following table:

TABLE I Yield strength Thickness of reference plate, Yield strength of reference plate inches factor p.s.i.

The percentage degradation of tensile strength or elongation is simply noted on the back of the plate, or a table or correlation curves such as FIGS. 3 and 4 are prepared. From the destructive test data collected from numerous test samples, it has been found that at maximum soundness there is a maxium difference between tensile and yield strength are equal. Between these limits the fraction of the difference between tensile and yield strengths at any given soundness is found to follow a determinable relationship regardless of the actual conditions utilized in the casting process. The tensile strength fraction curve presented in FIGURE 3 presents the percent of the difference between tensile and yield strength as ordinate versus increasing unsoundness as abscissa. Such a set of data or curves can only be used for a family of closely related alloys because of the differences other alloys exhibit in solidification range, temperature and mode of solidification unless coincidentally some diverse alloy exists which behaves in the same manner.

Referring now to FIGS. 1 and 2, a typical casting such as a missile fin 2 is cast with a coupon 4 in direct attachment to the casting. The mold is designed to surround the coupon with aluminum, cast iron, copper or other chills, not shown, along the top, bottom and longitudinal edges, to rapidly conduct heat away from the coupon to attain maximum soundness. The molten alloy is fed to the mold cavity through down sprue 6, elongate runners 8 and gates 10. A riser 12 is located at each gate 10 to provide gravity feed of molten alloy to compensate for contraction during solidification. The coupon 4 is preferably located on the casting between two risers 12 to assure absence of voids. Thus, the main casting and the coupon are substantially identical with respect to alloy composition and casting conditions such as molten metal, mold material, and pouring technique.

After solidification of the casting and removal of the mold, the casting is visually inspected for flaws and defects. Those castings which are acceptable are further processed, e.g. heat treated, in a conventional manner, but with the coupon attached to the main casting so that the mechanical properties of the coupon are affected by the treatment in substantially the same manner as the mechanical properties of the main castings.

After completion of all the heat treating processes, the casting is again visually inspected and rejected if it shows intolerable flaws. In addition to unaided visual inspection, one or more of the commonly used nondestructive insnec ion methods, such as fluorescent dye penetrant or X-ray inspection is used. These tests generally reveal any sharp or linear defects of a discrete or random nature which act as stress concentration points.

If the casting shows no flaws which are sufficiently serious to Warrant rejection, a relatively thin section of the skin of the main casting and coupon is removed, e.g. by chemical etching or chemical milling. The skin of the casting is denser and finer grained than the interior of the casting and prevents discovery of strength defects related to internal porosity. Generally about 0.001 to 0.010

inch of metal are removed from the surface by chemical milling but reliable results can be obtained with removal of about 0.0005 inch of metal. The milled casting is again processed by a similar dye penetrant inspection method used for the reference plates. Inspection of the casting at this point indicates defects of a more diffuse and general nature varying from microporosity to sponge shrinkage, which, while not necessarily intolerable, adversely affect the strength and related properties of the casting.

Microporosity so fine that it is not revealed by X-ray radiographic inspection is indicated 'by this method. Moreover, the intensity of the penetrant dye pattern which shows on the milled surface is a measure or indication of internal structure, microporosity, and soundness. The intensity of the dye penetrant pattern is measured quantitatively by comparing the visual appearance of the casting dye penetrant pattern with the previously established reference standard plates or reproductions. The casting may then be marked into areas of dye penetrant intensity.

The coupon is removed from the main casting and prepared into one or more standard tensile test bars which are tested for yield strength, tensile strength, percentage of elongation or other critical mechanical properties using known destructive testing methods. The test bar is cut so that it corresponds to the maximum soundness portion of the coupon. Thus, the test results indicate quantitatively the properties the main casting would have if it had maximum soundness. Of course, if the coupon has less than maximum soundness as shown by visual inspection of the dye penetrant pattern, this is taken into account in the calculation. For example, see Example 3 where the properties of a maximum soundness coupon are calculated from a casting of less than maximum soundness by dividing the casting properties by the degree quantity for the corresponding reference plate.

The mechanical properties of the coupon at maximum cast soundness can be utilized in several ways with the soundness of the casting to determine the quantity of the mechanical properties in various zones and thereby the acceptability of the casting. The predetermined acceptable mechanical property for the critical area can be related to the mechanical property at miximum cast soundness to establish a degree quantity. A corresponding minimum soundness quantity in terms of a reference plate is selected from the graph of FIG. 3 and Table I. This reference plate is then compared to the dye patterns in the critical area and the casting is rejected if denser dye patterns are present indicating a casting less sound and weaker than acceptable. Knowing the lowest numbered plate (maximum soundness) in any area, the appearance of dye patterns corresponding to a higher plate number determines a rejection of the casting.

It can also be determined whether any of the dye penetrant area correspond to mechanical properties below the limits set for the various critical zones of the casting such as A, B and C in FIG. 1. The areas are compared to the standard reference plates and the highest number plate is selected correspond-ing to the most intense dye penetrant areas (lowest soundness). The depreciation of mechanical property curves such as shown in FIG. 3 and Table I are then used to determine the minimum mechanical properties of each critical zone of the main casting.

The following specific examples are presented for illustrative purposes only, it being understood that equivalent steps or materials can readily be substituted within the procedure being described.

Example 1 An A357T6 aluminum alloy casting chemically milled to a degree of 0.002 inch is treated with a fluorescent dye penetrant and given a visual reading matching previously established reference plate No. 5 of the 0.50 inch thickness standards. The attached, maximum cast soundness,

7 200 mil coupon is removed and tested to give the following data:

Yield strength p.s.i 37,500 Tensile strength p.s.i 49,200 Elongation percent 20.0

Casting yield strength =0.93 37,500 p.s.i.=34,900 p.s.i. Casting tensile strength =0.500 (49,20037,500)+34,900:40,800 p.s.i. Provisional casting percent elongation The percent elongation calculated from FIG. 3 is a valid measurement only if the casting by coincidence, was given the same precipitation hardening heat treatment as the reference plates. Use FIG. 4 to adjust the provisional percent elongation for heat treatment. Find the point having the parameter of 42,800 p.s.i. yield (yield strength of 0.50 inch reference plate) and 4.0% elongation. Follow the slope until a yield strength of 34,900 p.s.i. (casting) is reached and read the final casting elongation of 7.4%. The mechanical properties of the casting are thus:

Yield strength p.s.-i 34,900

Tensile strength p.s.i 40,800

Elongation percent 7.4 Example 2 Referring again to FIGS. 1 and 2, a missile fin casting of A357T6 aluminum alloy has the following minimum specifications.

Yield strength p.s.i 40,000 Tensile strength p.s.i 50,000 Elongation percent 5.0

in the critcially stressed A areas and minimum specification of Yield strength p.s.i 30,000 Tensile strength p.s.i 40,000 Elongation percent 3 in the C areas of the casting. The casting and 200 mil coupon are chemically milled to remove the outer skin to a depth of 0.002 inch. Dye penetrant inspection shows the poorest reading in the critical A area to be comparable to N0. 2 reference plate of the 0.50 inch thickness standards and to No. 3 reference plate of the 1.00 thickness standards in zones C of the casting. From FIG. 3 and Table I, the corresponding fraction of maximum mechanical property figures are:

Critical Property Zone A Zone C Yield strength 0. 93 0. 84 Tensile strength 0. 800 0. 700 Percent elongation. 0. 55 0. 41

The coupon is destructively tested and shows 46,500 p.s.i. yield strength, 54,000 p.s.i. tensile (ultimate) strength and 10.0% elongation. From calculations similar to Example 1, the following data is obtained:

Critical Property Zone A Zone Casting, yield strength 43, 200 39, 100

Casting, tensile strength 49, :03 44, 30?

Casting, percent elongation The casting does not meet specifications in regard to the tensile strength of the critical zone A and therefore is not accepted for use.

The removal and destructive testing of each and every coupon provides the highest accuracy and assurance of discovery of faulty castings. However, there are cases where the penetrant pattern method of the invention can be performed without testing every coupon and statistically significant results are obtainable. For example, in the case of small castings poured at the same time from the same batch and processed together in under the same conditions, the method of the inventions provides accurate mechanical strength selection criteria by testing only a representative number of coupons.

Furthermore, there may be situations in which the casting is so small or the configuration so complex that it becomes unfeasible or impossible to place an attached coupon on the casting. If the conditions discussed above as to identity of batch and casting conditions are present, the test bar can be excised directly from one or more castings or the casting itself destructively tested. The following is an illustration of the modified procedure.

Example 3 A small turbine blade casting of A357T6 aluminum alloy has the following minimum specifications:

Yield strength p.s.i. 28,000 Tensile strength p.s.i 38,000 Elongation percent 10.0

A lot of castings have been poured from the same crucible of molten metal, have been heat treated together and all castings have been electrolytically polished to remove the surface to a depth of 0.0015 inch. All castings have been processed through the fluorescent dye penetrant inspection process. The reference plate number and the thickness standard is determined and recorded for each casting. The thickness standard of 0.20 inch applies to each and every casting. The reference plate numbers of the castings vary from 1 through 5.

Two castings rated With reference plate number I are selected for destructive testing. Coupons are exercised and tested and the average results are as follows:

Yield strength p.s.i 32,000 Tensile strength p.s.i 50,000 Elongation percent 15.0

By reversing the calculation process explained in Example 1 of the disclosure and using FIG. 3 and Table I, the mechanical properties of a test coupon, if it had existed, would be as follows:

Yield strength p.s.i 32,000 Tensile strength p.s.i 52,000 Elongation percent 20.0

Using this data, the next step is to calculate that ref erence plate number which is of borderline acceptability. Again, this is accomplished by reversing the calculation process explained in Example 1 of this disclosure. By calculation, the reference plate number of 2.5 is equated to mechanical properties of Yield strength p.s.i 32,000 Tensile strength p.s.i 47,000 Elongation "percent" 20.0

and is the basis for judging acceptability. All castings in the lot that have reference plate numbers of 2.5 or lower are accepted as meeting the minimum mechanical properties. Those castings having reference plate numbers higher than 2.5 were rejected.

It is apparent that this method provides a practical, nondestructive test giving reliable quantitative results with respect to various mechanical properties. While the method has been described with reference to a particular em bodiment, it is intended that the scope of the invention be limited only in accordance with the following claims.

What is claimed is:

1. A method of nondestructively testing a metal casting comprising the steps of:

preparing a series of cast sample plates from metal the same as being tested and decreasing in soundness from maximum cast soundness; chemically milling the surface of each plate and treating the milled surface with dye to establish standard dye patterns related to the soundness of the plate;

determining the depreciation factor of the mechanical properties of said plates corresponding to the various dye patterns on said plates;

casting a coupon of determinable soundness along with the main casting; removing a test bar from the coupon; destructively testing the test bar and measuring the quantity of a particular mechanical property at the determinable soundness of said coupon;

nondestructively measuring the soundness of the main casting in at least one critical zone;

comparing the soundness of said zone to said visual standard dye patterns to establish the level of depreciation of soundness of said zone and the corresponding predetermined depreciation factor for said mechanical property;

quantitatively measuring said mechanical property in the critical zone of the main casting by depreciating said quantity by said factor; and

grading the main casting in accordance with the quantitative measurement of said property.

2. A method of nondestructively testing castings for at least one particular mechanical property in accordance with claim 1, wherein the cast coupon has a portion of substantially maximum practicable attainable cast soundness an dthe coupon is cast and heat treated in attachment with the main casting.

3. A method of nondestructively testing castings for at least one particular mechanical property in accordance with claim 1, wherein at least a portion of the skins of the main casting and coupon are removed by chemical milling prior to nondestructively measuring the soundness of the main casting in the critical zone by dye penetrant inspection.

4. A method of nondestructively testing castings for at least one critical property in accordance wih claim 1, wherein:

the main casting is marked into zones of relatively common acceptable mechanical property; and,

the particular mechanical property is quantitatively measured for the least sound areas of each zone. 5. A method of nondestructively testing a plurality of similar metal castings for mechanical properties such as tensile strength, yield strength, and ductility comprising the steps of:

destructively testing sample castings made under controlled conditions to determine the correlation between depreciation of maximum cast soundness and the corresponding depreciation of each of the mechanical properties; casting a coupon having a portion of a maximized soundness with each of the plurality of similar main castings, each coupon being so attached to the main casting that it does not effect the quality of the casting and may be removed without damaging the main casting; treating each coupon with its associated main casting so that all major factors other than cast soundness which effect the strength of the main casting, effect the coupon to substantially the same degree;

inspecting the castings and rejecting those with serious flaws;

chemically treating the surfaces of the coupons and main castings to remove the outer skin;

nondestructively testing the soundness of each main casting by fluorescent dye penetrant inspection;

preparing a test bar, from each coupon, and destructively testing the maximized soundness portion of the bar for each of the mechanical properties;

using the determined correlation information, the determined soundness of each main casting, the known soundness of each coupon and the coupon destructive test data to quantitatively determine the minimum mechanical properties of each main casting in each of the several zones; and,

sorting those main castings which are acceptable with respect to all of the tested mechanical properties from those which are not acceptable.

6. A method of determining the acceptability of a metal casting by nondestructively quantifying the mechanical properties of critical areas of the casting utilizing a standard correlation of degree of mechanical property to soundness for castings of the same metal, comprising the steps of:

destructively testing sample castings of said metal made under controlled conditions to determine the correlation between the degree of the mechanical property at maximum cast soundness for sample castings of lower cast soundness;

casting a coupon of determinable soundness along with the main casting;

nondestructively measuring the soundness of the main casting in at least one critical area;

destructively testing the coupon to establish the quantity of the mechanical property at the determinable soundness and at maximum cast soundness;

establishing the degree of maximum mechanical property of the critical area by comparing the soundness of the area to the standard;

quantifying the mechanical property by a multiplication of the degree quantity by the maximum soundness quantity of the mechanical property; and

grading the main casting in accordance with the measurement of said property.

7. A method of determining the acceptability of a metal casting by nondestructively quantifying the mechanical properties of critical zones of the casting utilizing a correlation of degree of mechanical property to soundness for castings of the same metal, comprising the steps of:

destructively testing sample castings of said metal made under controlled conditions to determine the correlation between the degree of the mechanical property at maximum cast soundness for sample castings of lower cast soundness;

casting a coupon of determinable soundness along with the main casting;

destructively testing the coupon to establish the quantity of the mechanical property at the determinable soundness and at maximum cast soundness;

comparing the acceptable mechanical property of the critical zone to that of the maximum cast soundness of the coupon to establish a degree quantity;

converting the degree quantity to a soundness quantity by reference to the standard correlation;

nondestructively measuring the soundness of the casting in at least one critical zone; and

grading the main casting in accordance with the absence of areas less sound than the soundness quantity for the acceptable mechanical property in the critical zone of the casting.

8. A series of standard reference plates for use in the nondestructive testing of metal castings, comprising:

a series of plates which are graded in accordance with depreciation of mechanical properties, said plates varying in soundness from maximum cast soundness to the brittle soundness of the metal and said soundness being correlated with said depreciation of mechanical properties; and

each plate being chemically milled to a depth of 0.0005 to 0.010 inch and exhibiting dye patterns on the surface portions corresponding to microvoid channel entrances.

9. A series of standard reference plates according to claim 8, in which afiuorescent dye is present on the milled surface combined with an adsorbent.

10. A set of reproductions of the dye patterns of the standard reference plates defined in claim 8 for use in the nondestructive testing of metal castings.

11. A method of nondestructively determining the mechanical properties of a critical area of each of a series of metal castings, comprising the steps of:

removing the metal surface of the critical area of each casting Without plastically deforming the subsurface layers and treating the milled area With dye to establish dye patterns;

destructively testing a test bar of determinable cast soundness cast from the same melt as the main casting, during the same pour, for determining the quantity of a desired mechanical property at maximum cast soundness;

providing a series of standard reference dye patterns for the same metal cast to have various solidification rates and integrity of cast structure, said dye patterns being correlated to the degree of depreciation in mechanical properties from the mechanical properties at maximum cast soundness by destructively testing the casting portion Which produces each dye pattern;

comparing the dye patterns of each casting with said standard reference dye patterns for the metal of the the casting; and,

determining the desired mechanical property of the 12 critical area from the correlated degree of depreciation of mechanical properties and said determined quantity for the desired mechanical property at maximum cast soundness. 12. A method as described in claim 11 wherein said castings are aluminum base alloy castings.

References Cited UNITED STATES PATENTS 4/1955 De Forest 73-l04 9/1960 Switzer 73104 FOREIGN PATENTS 8/1958 U.S.S.R.

OTHER REFERENCES JERRY W. MYRACLE, Primary Examiner US. or. X.R. 

